Image-forming materials of the above-described kind have so far been used as so-called "free radical photographs", the optically exposed areas of which are visualized by imagewise exposure, for many photographic purposes. On the other hand, heat sensitive recording has been performed for a long time according to various methods.
Materials particularly effective in forming light images are those utilizing radical coloration of various leuco dyes in tints of their corresponding dyes through the reaction with a photo-oxidizing agent (as described, e.g., in Photo. Sci. Eng., vol. 5, pp. 98-103 (1961), JP-B-43-29407 (The term "JP-B" as used herein means an "examined Japanese patent publication"), and U.S. Pat. Nos. 4,271,251, 4.298,678 and 4,622 286).
As for the heat sensitive recording, heat-sensitive recording materials utilizing, e.g., combinations of electron-donating dye precursors with electron-accepting compounds are disclosed in JP-B-45-14039 and JP-B-43-4160, and those utilizing diazo compounds are disclosed in U.S. Pat. No. 4,650,740, and so on.
In recent years, these image recording materials have come to be used in various areas of recording, such as facsimile, printers, labels, etc., in addition to photographic applications, so they have enjoyed an increasing need. However, because of their susceptibility to light and heat, they have a substantial defect in that they generate light fog and heat fog when exposed to ordinary room light, sunlight or white light even after the formation of images therein. Therefore, it is difficult to handle image-forming materials of the foregoing kind.
In order to retain the images which have been formed, post-development of colors which render the images obscure during storage must be avoided.
In the image-forming systems, it has already been known that primary images can be retained by applying a solution comprising a reducing agent, such as a free-radical trapping substance (e.g., hydroquinone), to materials, which have finished image formation, using a spraying or dip coating technique. This procedure, although advantageous for preserving or fixing the primary images, brings about an inevitable decrease in workability and operation facility, because it involves a wet process. On the other hand, materials which can be processed only by light alone, that is to say, images are formed by UV irradiation and fixed through the activation of photoreducing substances by exposure to visible light, as embodied in Dylux made by Du Pont, are on the market today. In this process, however, the material is in a sole possession of the apparatus during the light exposure performed twice, which entails work such as replacement of spectral filters. Accordingly, insufficient processing speed occurs in some uses. In addition, JP-B-43-29407 describes that thermal fixation after imagewise exposure is effected by incorporating a reductive thermo-fixing agent in a binder solution together with a leuco dye and a photo-oxidizing agent, or by providing the last coat of a thermo-fixing agent on a light-sensitive layer. However, this system undergoes deterioration in sensitivity over time because the fixing agent is present in the neighborhood of the light-sensitive components (a leuco dye and a photo-oxidizing agent). Therefore, such a system is also undesirable.
In producing the foregoing image-forming materials, in general, ingredients including at least a leuco dye and a photo-oxidizing agent are homogeneously dissolved in a volatile organic solvent, and the resulting solution is coated or spread out on a support, such as paper, a plastic film, etc., or a support is dipped in the resulting solution, and then the solvent is evaporated to removal by drying. Under these circumstances, it is necessary to take effective measures to prevent the manufacturing equipment from being exposed to such a volatile organic solvent as described above. Accordingly, the image-forming materials of the above-described kinds have disadvantages in terms of both safety and cost.
In heat-sensitive recording systems, on the other hand, recording methods utilizing a color-producing reaction through the coupling of a diazo compound with a coupler and a photo-decomposibility to ensure fixability are disclosed [e.g., in JP-A-57-123086 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), U.S. Pat. No. 4,400,456, JP-A-57-142636 and JP-A-57-192944]. Therein, however, fog and the like tend to occur upon storage prior to recording because reactive components (including a diazo compound, a coupler, a base, etc.) are incorporated in the same layer. Therefore, the results obtained by such methods are not wholly satisfactory. Moreover, heat-sensitive recording materials containing one of the foregoing reactive components in a microencapsulated condition are disclosed, e.g., in JP-A-59-190886, etc., which indeed show excellent image-keeping quality and high fixability, but they do not necessarily possess sufficient keeping quality before the recording processing since the diazo compound is used. Therefore, it is desired that more stable substances should be selected for not only the diazo compound but also the coupler, the base, etc.